The present invention relates generally to vehicle suspension control systems, and more particularly, to low force actuators for use in such vehicle suspension control systems.
In general conventional vehicle suspension control systems are either passive, active, or semi-active. Semi-active and passive systems are, by definition, incapable of providing active forces, and in the presence of stiction, are inherently "locked up" for small disturbances. Passive practice is to experimentally tailor the suspension parameters to each vehicle model. The passive system is tuned to the perceived requirements of the potential customers. Sports cars are stiffly sprung and highly damped. Luxury cars are softly sprung and lightly damped. Passive implementations have fixed parameters and represent highly compromised designs. Semi-active practice appears to be evolving to the use of closed loop controllers to adjust the damping characteristics of the suspension. These systems provide significant improvement in ride and holding performance except for "smooth roads". The "smooth road" performance is limited by suspension stiction and valve transients in the damping adjustment mechanism that can occur (dependent on mechanism design) when the shock relative velocity changes sign.
Active suspension systems as currently implemented are high pressure, high flow hydraulic systems. Practice has been to build systems capable of lifting a corner of the vehicle off the ground. These systems are high force, high bandwidth systems and hence high power (25 horse power is typical for a large vehicle). Active systems universally exhibit harshness and degraded ride performance on "smooth roads". They are implemented using high pressure and flow hydraulic actuators. These systems are expensive and power inefficient. They take up a lot of volume, since they require accumulators, tubing, pumps, valve, and hoses, and the like.
For active systems, prior practice has been to use high force actuators (3000-5000 N) capable of literally lifting a corner of the vehicle off the ground. At the dynamic ranges typical of low cost control systems (100:1) these systems simply do not have the resolution required to control the small disturbance inputs (less than 200 N) that occur most of the time when driving over "smooth" roads.
The harshness is caused by the high power, high force levels that are controlled by control systems that use inexpensive sensors (low dynamic range, nonlinear). This practice results in degraded ride on "smooth roads" because of the dynamic range limitations of the systems (if you must control 1000's of Newtons it's hard to control to 10's of Newtons with inexpensive controls). High pressure high flow hydraulic systems are never going to be inexpensive and will always require a lot of power.